Ink jet printing is accomplished by ejecting ink from a nozzle toward paper or another print medium. The ink is driven from the nozzle toward the medium in a variety of ways. For example, in electrostatic printing, the ink is driven by an electrostatic field. Another ink jet printing procedure, known as squeeze tube, employs a piezoelectric element in the ink nozzle. Electrically-caused distortions of the piezoelectric element pump the ink through the nozzle and toward the print medium. In still another ink jet printing procedure, known as thermal or bubble ink jet printing, the ink is driven from the nozzle toward the print medium by the formation of an expanding vapor phase bubble in the nozzle. These various printing methods are described in "Output Hard Copy Devices," edited by Durbeck and Sherr, Academic Press, 1988 (see particularly chapter 13, entitled "Ink Jet Printing").
Preferably, an ink jet printer is capable of printing with colored ink, such as magenta, cyan and yellow, as well as black ink. When two colors are printed side by side, particularly when black ink is printed next to any other colored ink, the colors can "bleed" into one another. "Bleed" is defined as the migration of one ink color into a region of another ink color, particularly when black ink moves into a region of any other color. It is desirable to have a clean, crisp border between areas of two different colors. When one color bleeds into the other color, the border becomes irregular and ragged. Bleed is particularly undesirable when black ink is printed next to a light color ink, such as yellow.
Numerous methods have been developed in an attempt to reduce or eliminate the bleed between different colors of ink, particularly the bleed between black ink and colored ink.
One method to reduce bleed between inks is to incorporate one anionic ink and one cationic ink as disclosed in European Patent 633,142, Stoffel, et al., published Jan. 11, 1995. Both the anionic and cationic inks are aqueous solutions and contain a colorant which may be either a pigment or a dye. In one of the two inks, a polymer must be added which is of the same ionic character as the ink in which it is incorporated.
Cationic dyes are also disclosed in U.S. Pat. No. 5,198,023, Stoffel, issued Mar. 30, 1993. In this patent, a cationic yellow dye is used with an anionic black dye. Bleed is further reduced by adding a multivalent precipitating agent to the yellow ink. This multivalent precipitating agent is typically a multivalent salt, such as calcium chloride, magnesium chloride and aluminum chloride.
Bleed can also be alleviated by using pH sensitive dyes. U.S. Pat. No. 5,181,045, Shields, et al., issued Jan. 19, 1993, describes the use of a dye which is rendered insoluble by contacting it with another ink of the proper pH. This reaction occurs at the border of the two inks and is distinguished from systems where the pH of the paper is used to render the dyes insoluble. The pH of the second ink can be either higher or lower than that of the first ink. However, the pH difference should be greater than one unit. The '045 patent discloses dyes with proper pH. U.S. Pat. No. 5,320,668, Shields, et al., issued Jun. 14, 1994, which is a continuation in part of the '045 patent, discloses not only dyes but inks containing either pigments or dyes.
Color bleed is controlled by employing zwitterionic surfactants or ionic or non-ionic amphiphiles according to the teachings of U.S. Pat. No. 5,106,416, Moffat, et al., issued Apr. 21, 1992. The inks described contain one or more cationic dyes.
Bleed resistance is increased in dyes by counter-ion substitution in U.S. Pat. No. 5,342,439, Lauw, issued Aug. 30, 1994. A dye having one or more sulfonate or carboxylate groups is provided with a counter-ion comprising an amine, which is used for its surfactant properties. Such a dye is produced in an ionic exchange process.
The use of precipitating agents is taught in U.S. Pat. No. 5,428,383, Shields, et al., issued Jun. 27, 1995. Color bleed between two ink compositions is controlled by incorporating a precipitating agent in the second ink which precipitates the first ink coloring agent. When the two ink compositions contact each other on the paper, a precipitate is formed which prevents migration and color bleed problems.
In U.S. Pat. No. 4,694,302, Hackleman, et al., issued Sept. 15, 1987, the ink includes a reactive species which forms a polymer when the ink hits the paper. The reactive species either reacts with a component in the substrate, i.e., the paper, or alternatively reacts with a material which is applied to the substrate before the ink is applied.
U.S. Pat. No. 5,476,540, Shields, et al., issued Dec. 19, 1995, teaches the use of gel forming inks to alleviate bleed. In such a system, one ink contains a gel forming species and the other ink contains a gel initiating species, typically a protonated tertiary amine. When the two inks come in contact with each other, gel is formed, thereby preventing movement of the coloring agent.
Micro-emulsions comprising water insoluble black dyes are also used to prevent bleed between the black ink and the colored ink. Such inks are taught in U.S. Pat. No. 5,342,440, Wickramanayake, issued Aug. 30, 1994, and U.S. Pat. No. 5,226,957, Wickramanayake, et al., issued Jul. 13, 1993. In each case the black dyes are water insoluble. They are used in conjunction with colored inks that contain water soluble dyes. The water insoluble black dyes will not migrate through the water based color inks and, thus, bleed is prevented.
Bleed is also controlled by adding additional agents to the ink composition. For example, in U.S. Pat. No. 5,196,056, Prasad, issued Mar. 23, 1993, a bleed retarding agent which has a polar portion and a non-polar portion is added to the ink. A particularly preferred bleed retarding agent is 2-(2-butoxyethoxy)ethanol. In U.S. Pat. No. 5,160,372, Matrick, issued Nov. 3, 1992, an ester or amide diol is added to the ink to improve the penetration of the ink into the paper. This also provides rapid drying.